Redundant DSL Questions?!

Re: Redundant DSL Questions?!

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There are many issues to be resolved here. First of all, let me
mention that QuickDNS won't respond properly on any address other
than the primary address (as configured in the TCP/IP control panel).
This will be addressed in QuickDNS 3, due out later this year.

Aside from that limitation of QuickDNS, which can be overcome with
sufficient ingenuity, let's look at your two connections.

What happens if line Q goes down? A web visitor tries to pull up your
web site. Since your DNS servers don't know that line Q is down,
they're still giving out both addresses, even if you use QuickDNS'
fault tolerance. So the web visitor might get 208.45.152.132. Since
the routers of the world don't know that this address can be reached
through line G, the request is routed to the downed line Q, where it
stops.

This can be solved, in theory, using BGP and the cooperation of one
or both ISP's. However, with DSL lines, such cooperation is highly
unlikely.

One of your best options is to have two different versions of your
domain file. Have each DNS server only respond on one IP address -
one on a Q address and one on a G address. The server on the Q
address has a version of the domain that uses the Q addresses of your
web/mail/etc servers. Similarly, the server on the G address has a
version of the domain that uses the G addresses.

Then if line G goes down, DNS queries for the domain only reach the Q
server, which gives out the Q address of the web server. Of course,
resolvers that have the G addresses cached will cause problems for
some potential visitors, but this can be mitigated by setting the
TTL's of the A records for the web server to low values - 300 seconds
(5 minutes) or less.

Also, visitors who are in the midst of browsing your web site when a
line goes down will be cut off until they quit and relaunch their
browsers; there's nothing you can do about this, unless you start
using BGP.

Another good option is to colocate your server somewhere that does
use BGP. This way, you get all the benefits of redundant connections,
without the hassle of setting it up and maintaining it yourself.
____________________________________________________________________
Chris Buxton cbuxton@menandmice.com
Men & Mice http://www.menandmice.com
Makers of: QuickDNS Pro

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